1. Field of the Invention
The present invention relates, in general, to a torque control method for a high-speed Switched Reluctance Motor (SRM), and, more particularly, to a torque control method for a high-speed SRM, which compensates for the positive torque of an active phase (A phase) of two phases based on a negative torque attributable to an inactive phase (B phase) during a compensation control enable interval (ENA) ranging from a time point at which the active phase (A phase) is turned on to a time point at which the tail current of the inactive phase (B phase) remains, in order to remarkably reduce a high torque ripple that occurs in an overlap area in which the phase switches from an active phase to a subsequent excited phase, in consideration of the non-linear characteristics of a high-speed SRM, and thus to improve drive efficiency.
2. Description of the Related Art
Typically, a Switched Reluctance Motor (SRM) is a kind of reluctance motor which includes principal components such as a poly-phase stator, a rotor, and a position detection unit. The poly-phase stator allows armature coils to be wound therearound and produces a magnetic force. The rotor is rotated by the magnetic force produced by the stator and a magnetic force produced according to the relative position of teeth. The position detection unit is provided with a position detection sensing unit and a sensor plate and is configured to detect the position of the rotor by sensing a position detection pulse with a predetermined angle resolution as the position of the rotor varies. A plurality of teeth are symmetrically formed in the rotor, the armature coils are symmetrically wound around the poly-phase stator, and the position detection sensing unit detects the position of the rotor, and outputs the position detection pulse, so that poly-phase armature coils are sequentially driven in synchronization with the position detection pulse.
Such an SRM controls power that is supplied to the armature coils wound around the poly-phase stator by using switching elements. In this case, as an input pulse signal is applied to the control terminal of the switching elements in synchronization with the position detection pulse output from the position detection unit, an excited state between the rotor and the stator sequentially varies, and thus a forward rotation torque corresponding to the input pulse signal can be generated on the rotor by a magnetic suction force. Further, when a specific excited state does not vary, the rotor can be stopped at a predetermined position. A reverse rotary force (reverse torque) can be produced by controlling the phase of the input pulse signal applied to the switching elements on the basis of the maximum inductance shape. In this way, since various types of drive control for the SRM are possible, such an SRM has been widely and usefully used in a variety of application fields. In particular, an SRM has been frequently applied to and used in high-speed rotary systems such as blowers, compressors, and pumps, thanks to its compact size and excellent system efficiency.
However, the greatest disadvantage of the high-speed SRM is that a torque ripple is greater than that of other motors. In particular, in the SRM, a high torque ripple appears in an overlap area (a current interval) in which a phase switches from an active phase to a subsequent excited phase.
In order to solve this problem, a plurality of conventional technologies related to a torque control method for an SRM have been proposed. One of these technologies is Korean Patent Registration No. 976029. This conventional technology relates to a Direct Instantaneous Torque Control (DITC) system for an SRM using a 4-level converter, and discloses a DITC system for an SRM, which includes a torque estimation unit, a hysteresis control unit, a switching table unit, and a 4-level converter unit. The torque estimation unit estimates torque based on a three-dimensional (3D) Look-up Table (LUT) using detected phase current and the position of a rotor. The hysteresis control unit selects switching rules according to the position of the rotor, and generates the state signals of an input phase and an output phase based on hysteresis control corresponding to a difference between the estimated torque and a reference torque (torque error). The switching table unit converts each state signal into a switching signal composed of four operating modes (mode 1, mode 0, mode −2, and mode 2). The 4-level converter unit controls the operations of the SRM in such a way as to supply a power source voltage to the SRM in mode 1, return the current of coils to a power source in mode 0, supply the power source voltage and a boost capacitor voltage to the SRM in mode 2, and recover the energy stored in the coils to a capacitor in mode −2.
The above conventional technology is advantageous in that relatively smooth torque output can be derived using basic control principles, but has limitations in that complicated switching rules are required to generate a smooth torque in an overlap area, and control performance is determined based on the switching rules determined in this way. Further, there is another problem in that when a DITC technique is used, an additional current controller must be employed.
Another conventional technology, that is, Korean Patent Registration No. 228695 discloses an SRM control method which optimizes the turn-on and turn-off times of the switches of armature coils, thus effectively reducing a torque ripple. This patent is characterized in that an SRM has a turn-on duration corresponding to a predetermined period of time and is operated within the upper limit of delay time on the basis of an LUT which receives a position signal obtained by detecting the position of a rotor and in which speed-based turn-on delay time data preset according to rotation speed is recorded.
However, the conventional technology is still problematic in that a torque ripple in an overlap area is not especially taken into consideration, and thus the influence of the torque ripple that occurs due to tail current in the overlap area cannot be excluded.
Meanwhile, various methods using a torque sharing function have been proposed as a torque control method for an SRM. Graphs of the command torque of a 2-phase SRM using such a torque sharing function, and the command torque and the command current of each phase depending on the mechanical structure of the motor are shown in FIG. 1. An overlap area in which both the 2-phase torques of an active phase and an inactive phase are generated corresponds to an interval in which the rising sections of inductance overlap due to the mechanical shape of the motor. In this interval, the sum of torques of two phases determines the total torque.
The torque ripple appearing in the high-speed operation of the SRM when such an existing torque sharing function is used is shown in FIG. 2. As shown in FIG. 2, current required to generate a command torque in the high-speed operation area starts to be extinguished at a turn-off angle. However, since a torque overlap interval is very short, the current cannot be completely extinguished during the interval, and then tail current is generated. Because this tail current exists in the falling section of inductance, a negative torque is generated, and a high torque ripple occurs for the entire torque due to the negative torque.
That is, the torque control method for the SRM using the existing torque sharing function is simple, but is problematic in that the torque ripple cannot be mitigated at the start and end portions of the overlap area in high-speed operation mode.